Maintaining ink composition in an ink jet printer within an operative range is a significant problem. As the ink solvent evaporates, the concentration of nonvolatile components increases to a level where the printer begins to fail. Typically, this problem is solved by replenishing from separate supplies the ink concentrate and the solvent. This is not attractive because of the expense of shipping two supply items rather than one to a world market. U.S. Pat. Nos. 3,761,953, 3,930,258, 4,121,222 and 4,130,126 show examples of printers having dual replenishment supplies--ink concentrate and solvent.
Another solution to the problem is to use a single replaceable ink reservoir or ink bottle. Because of the evaporation rate in the ink recirculation system, the ink composition becomes more concentrated. The ink bottle must be changed whenever the ink concentration and thus the ink viscosity become too high for print operations. Ink remaining in the bottle, when it is discarded, is lost. U.S. Pat. No. 3,929,071 shows such a printer where ink bottles are replaced even though they are not empty.
The IBM 3890, a bank check processing machine, uses a single replenishment fluid in an ink jet printer. There is a permanent ink reservoir, and replenishment ink is supplied from a separate bottle. The concentration of nonvolatile ink components in the ink composition settles within an operative range because the 3890 has a narrow print rate range. The single type of print usage allows the ink concentration to remain within the operative range for the printer even though the evaporation rate of the ink recirculation apparatus is not controlled.
The problem then is to recirculate ink in a printer having a wide range of print rates while minimizing the ink evaporation rate so that the ink may be replenished with a single fluid.